Several pumping circuits or dispositions are known that correspond to such a leak detection system.
Firstly there is a circuit in which the pre-evacuation pump and the primary pump are distinct, each generally being constituted by a vane pump.
In other assemblies, the pre-evacuation pump for the test chamber and the primary pumping system situated at the delivery of the secondary mechanical pump are constituted by a common pump with a valve being disposed between said common pump and the delivery of the secondary pump, or else a three-way valve system is provided for enabling the common pump to be connected either to the delivery of the secondary pump, or else to the first duct leading to the test chamber.
Under these circumstances, prior assemblies are capable of operating either in direct flow or in counterflow, or else of operating selectively in direct flow or in counter flow, For direct flow operation, the suction of the secondary pump is connected to the test chamber via an inlet valve. For countercurrent operation there is a connection including a valve suitable for connecting the delivery of the secondary pump to the test chamber.
Which ever configuration is used, it takes a long time to pre-evacuate the part under test when its volume is large. At less than 10.sup.-1 mb, the flow rate through a primary vane pump is relatively low. Unfortunately, the valve connecting the chamber under test to the secondary pump can be opened only after the press has dropped down at least to 10.sup.-2 mb.
In addition, throughout the possibly long period that the chamber is being pre-evacuated under molecular conditions, there is always the risk of the chamber being polluted by oil from the primary pump.
A helium memory phenomenon exists in configurations having a single primary pump, with this phenomenon being due to the vane primary pump whenever a prior test has caused a large leak to be detected. This phenomenon disturbs the results of the following tests.
The object of the present invention is to mitigate these drawbacks and it also has the advantage of reducing the signal "background noise" that exists in such a detection system.